High Dose Rate (HDR) Brachytherapy (BT) is a form of radiation treatment in which a sealed radioactive source is introduced into the anatomy of a patient to be treated through an applicator. Recent advances in 3D image-based treatment planning for intracavity cancer BT, including, but not limited to, cervical cancer BT, have improved clinical outcomes. Effectiveness and accuracy of the treatment depends upon a fixed applicator position. Unless computerized tomography (CT) or magnetic resonance (MR) scanners are installed in the treatment room in which HDR BT is occurring, a patient must be transferred out of treatment room in order to perform 3D image guidance.
Even though 3D image guidance has improved clinical outcomes in BT applications, there are still some shortcomings. One of the challenges still faced when using 3D image guidance is when the applicator is displaced from the specific placement on or within the body of the patient. Applicator displacement frequently occurs when patients are transferred between the treatment room and the imaging room in instances wherein CT or MR scanners are not found in HDR BT operating rooms. In addition, displacements occur between applicator insertions, during patient relocations, and during prolonged periods between imaging and treatment. Further, in gynecologic interstitial BT, needles or catheter displacements were caused by patient movement, or needles inadequately secured to the template.
3D imaging based, conformal BT has a tight target (e.g. high risk CTV) coverage with high dose-gradient. Therefore, a small applicator displacement can cause a significant radiation dose changes between intended doses and delivered doses. These displacements resulted in shifts of source dwell position relative to the target structures and organs at risk (OAR), altering the delivered dose. For example, De Leeuw et al. reported an average displacement of 3-4 mm in the cranial-caudal direction resulting in an average dose change of 4% to the rectum. Gerszten et al. reported up to 12 mm displacement, when using an applicator-immobilization device. Kim et al. reported up to 2.4 mm displacement when using an in-house developed applicator-immobilization system.
Given the potential displacement, there is a need to monitor the displacement. In some instances, to monitor the potential applicator displacement, an X-ray is taken before and after a patient transfer to 3D image (CT or MRJ) scan. However, 2D X-ray is limited to show applicator displacement. In addition, no real-time monitoring is possible, since fluoroscopy imaging would require such a significant radiation dose to which a patient cannot be used to do real-time monitoring for an applicator. It has been reported 3 mm displacement of applicator causes 10% dosimetric error in HDR or Pulse-Dose-Rate BT. Therefore it is desirable to have some means to ensure no motion of the applicator occurs.
Several techniques and products have been used to address applicator or needle displacement. The Zephyr system (DIACOR Inc., Salt Lake City, Utah) uses an air-bearing technique whereas the MedTrack system (MedTrack, LLC, Madison, Wis.) uses ceiling rails. Needles can be secured with a variety of techniques, including buttons or adhesive. However, fixation techniques do not address applicator and needle immobilization. If an immobilization device is rigidly attached to a transfer-table, incidental patient motion can harm the patient. Therefore immobilization devices should be attached to the patient to follow their movement. Even with an immobilization device, applicator displacement of up to 2.4 mm (10) and 12 mm (4) has been reported. The displacement can have an impact on the treatment. For example, the dosimetric impact of applicator shifts was reported to be an average change of 3-5% on rectum per mm of applicator displacement in cranial-caudal direction.
In gynecological HDR BT, applicator displacement can be measured using reference points and markers on the bony anatomy. Hoskin et al. investigated applicator displacement using the distance between the ovoid source and bladder or rectal reference points. Significant applicator displacement was observed in the cranial-caudal direction relative to ICRU bladder point. Small applicator displacement relative to ICRU bladder point (median: 2 mm, range: 0-10 mm) was observed in anterior-posterior direction. These displacements were attributed to placement techniques and vaginal packing. Similar studies measured applicator displacement using a set of markers placed on the bony anatomy and dummy marker reference points inserted into the applicator. Bahena et al. investigated applicator displacement of the ring and tandem applicator relative to the bony anatomy. Using anterior-posterior and lateral orthogonal images, the applicator position was established using 4 bony landmarks of the pelvis and dummy marker reference points on the applicator. The reproducibility of the applicator position was measured by comparing the applicator positions in the subsequent implant to the position in the first implant. A displacement of 6.5, 5.9 and 7.7 mm was observed in the superior-inferior direction, lateral direction and anterior posterior direction respectively, which was due to changes of both tumor volume and anatomy during the course of treatment.
Pham et al. evaluated the change in the applicator position during multiple HDR insertions using an unfixed tandem and ovoid applicator system. The changes in the applicator position were measured relative to the patient's bony pelvic landmarks. The authors observed an average longitudinal displacement of 3 mm for the tandem and 2 mm for the Ovoids. Thomadsen et al's. study used a fixed HDR applicator system and observed an average shift of 1.7 mm relative to the bony pelvis even though the applicators were fixed to the treatment table. Several studies have examined the displacement inherent in gynecologic interstitial BT. Mikami et al. measured the distance between the applicator tip and the reference point of center of gravity of the three implanted titanium markers. Significant applicator displacement was observed in the caudal direction (median: 1 mm, range: −6 to 12 mm). Shukla et al. noted a 17 mm applicator displacement in the caudal direction. Using a template like the Syed-Neblett system, Damato et al inserted dummy markers into the applicator to identify the tips on computed tomography (CT) scans. Two CT scans of the pubic symphysis were used for rigid registration of the applicator shift, where the authors observed needle displacement up to 20.0 mm in the cranial direction and 16.3 mm in the caudal direction.
Current applicator tracking techniques are not able to provide continuous and real time measurements while applicator displacement can also be caused by incidental patient motion after the 3D scan and during treatment delivery. Currently, no commercial system is available for continuous monitoring applicator motion. Those commercial solutions available only takes snapshots of applicator using either CT or C-arm based X-ray systems, so they cannot be used for continuous monitoring due to high imaging dose.
Therefore, there is a need for a system and method for providing continuous and real time measurements of applicator displacement. Further, there is such a need for such a system and method that does not require additional imaging that does not expose the patient to high imaging dosage.